1. Field of the Invention
The present invention relates to a power-line-operated inverter-type power supply operable to series-drive a high-Q parallel-loaded resonant L-C circuit, especially in applications where the load is of a type that must be conditioned before becoming conductive.
2. Background Considerations
Normally, a load that must be conditioned before becoming conductive is one that depends on electron emission for its conductivity. Such loads include fluorescent lamps and vacuum tubes.
In such a load, a cathode must be heated to incandescence before electron emission starts; and this heating proccss is apt to take on the order of one to several seconds. Thus, aside from the relatively modest amount of power needed to accomplish the conditioning, such a load is substantially a non-load until its cathode has reached incandescence.
In many applications of inverter-type power supplies, such as in inverter-type ballasts for fluorescent lamps, or in inverter-type microwave oven power supplies, it is often desirable to power such a load by way of having it parallel-connected across the tank capacitor of a high-Q resonant L-C circuit--with this L-C circuit itself being series-connected directly across the output of the inverter.
However, when such a high-Q series-resonant L-C circuit is not loaded, it acts substantially as a short circuit; which, during even a brief period (such as during the one or two seconds it takes for the load to become conductive), is apt to bring destruction to the inverter and/or to the L-C circuit itself.
One way of preventing such destructive overload is that of connecting in parallel with the load a voltage-limiting means (like a Varistor) characterized by: (i) not conducting at the highest magnitude of voltage normally present across the load when it is conducting; and (ii) conducting heavily at a voltage of somewhat higher magnitude than that.
However, in many applications, due to the very significant amount of energy that must be absorbed by this voltage limiting means, even if the required conditioning period is only a couple of seconds in duration, the effective cost associated with such a method of preventing destruction of inverter and/or L-C circuit is very high.
Also, the potential inefficiency involved--as for instance in a situation where the load is simply removed from the L-C circuit--can represent a major obstacle to designing a fully functional product.